1. Field of the Invention
The invention relates to a wireless communication system.
2. Description of the Related Art
A multi-channel system capable of wireless communications at two or more frequencies has been introduced as one type of wireless communication system for a vehicle. The wireless communication system has a transmitter and a receiver, and is able to conduct wireless communications at two or more frequencies, between the transmitter and the receiver. In the multi-channel system, the transmitter has an operating button for switching the transmission frequency. For example, the frequency can be changed when the user presses the operating button for a short time, and the frequency that has been used continues to be used when the user presses the operating button for a long time.
If noise (interfering waves) occurs during communications at a certain frequency f1 between the transmitter and the receiver, and at least a part of the frequencies of the noise is equal to or overlaps the frequency f1, the communications between the transmitter and the receiver may not be established. With the introduction of the multi-channel system, even in the case where the communications between the transmitter and the receiver are not established, the transmitter switches the transmission frequency to a frequency f2, and the receiver switches the receiving frequency to the frequency f2, so that communications between the transmitter and the receiver can be established. For example, the multi-channel system is introduced as a remote keyless entry (RKE) system. In this case, if a door does not open even though an operating button on the transmitter is pressed, the user can change the transmission frequency by pressing the operating button of the transmitter for a short time, and can also change the receiving frequency of the receiver. With the transmission frequency and receiving frequency thus changed, the user is able to open the door by pressing the operating button again.
The wireless communication system is provided with a fixed period of time for which communication errors are ignored. The fixed period of time may be called “no-radio-wave timer” or “mask time”. The transmitter sends substantially the same signal a plurality of times. Even in the case where a communication error occurs at a certain point in time, and the receiver fails to receive a transmission signal from the transmitter, the receiver can receive the signal transmitted from the transmitter at the next point in time if the no-radio-wave timer is started. For example, the receiver starts the no-radio-wave timer after confirmation of a certain frame, so as to save the next frame from a receiving error. The confirmation of the frame means that the frame can be successfully received. If the no-radio-wave timer is not started, the receiver continues to generate a signal indicative of no signal (the absence of signals) when a communication error occurs.
While the no-radio-wave timer is counting the set time, the receiver cannot change the standby frequency (or receiving frequency). This is because the no-radio-wave timer is started so as to save the signal transmitted from the transmitter from communication errors. Accordingly, even if an operation to change the frequency is performed by pressing the operating button on the transmitter for a short time, the no-radio-wave timer is in the middle of counting on the receiver side after confirmation of the frame immediately after the operation. Therefore, if the transmitter sends a signal at a frequency to which the transmission frequency has been changed, the receiver may not be able to receive the signal since the no-radio-wave timer is in the middle of counting.